Illite occurrences related to volcanic-hosted hydrothermal mineralization in the Biga Peninsula, NW Turkey: implications for the age and origin of fluids

A different approach to investigate the origin of fluids, temperature conditions, age of hydrothermal activity of mineralization in the Biga Peninsula, (Koru, Tesbihdere and Kumarlar) employed mineralogical (illite Kübler Index, b cell dimension, polytype) and geochemical (major, trace/REE, O-stable isotope and Rb/Sr dating) methods. The Kübler Index (KI) values of illites indicate different temperature conditions, such as low temperature (high-grade diagenesis) for Koru deposit, and high temperature (anchizone) for the Tesbihdere and Kumarlar deposits. The textural, mineralogical and geochemical data from illites show that these have potential for estimating the age of hydrothermal activity and fluid characteristics. Both mineralogical (high grade diagenetic to anchizonal KI, 1M polytype, low d values) and geochemical (similar major and trace element composition to host-rocks, low octahedral Mg + Fe contents, oxygen and hydrogen isotope composition) data are compatible with commonly known hydrothermal illites. Stable isotope data of illites are well matched to similar data from fluid inclusions, which indicate mainly magmatic fluids. The Rb/Sr age (22.4 ± 2.3 Ma: latest Oligocene and lowest Miocene) of the illites coincides with plutonic intrusions that are the main instigators of hydrothermal activities related to the extensional tectonic regime in the Biga Peninsula. The mineralogical and geochemical data of illites have some important advantages with respect to the use of fluid inclusions in determining δD of hydrothermal fluids thereby leading to better understanding ore-forming hydrothermal conditions